Hydraulic control system with priority flow control

ABSTRACT

A hydraulic control system for controlling the flow between a reservoir, a pump having pressure flow compensating means associated therewith and first and second flow control means for controlling flow to first and second fluid operated devices is disclosed herein. The hydraulic control system includes a priority flow control valve which maintains a priority of flow from the pump to the first flow control means based upon the amount of flow required as well as the pressure requirements. The priority flow control valve has a valve spool which is positioned as a function of a biasing spring, as well as the pressure of the fluid from the pump to the first flow control means and the pressure of the fluid in the first fluid operated device. The system is particularly adapted for functions requiring higher pressure than low stand-by system pressure and where one of those functions require priority over the other.

BACKGROUND OF THE INVENTION

The present invention relates generally to hydraulic control systemsthat form part of a vehicle such as an agricultural tractor. Moreparticularly, the present invention relates to a control system thatincorporates a pressure-flow compensating pump for deliveringpressurized fluid to a plurality of fluid operated devices with apriority flow control valve incorporated therein that establishespriority of flow to a first fluid operated device whenever the pressureor flow requirements for that device are below a certain level.

Hydraulic control systems for controlling various functions that formpart of a vehicle such as a tractor have been in existence for manyyears. Usually the hydraulic control system incorporates one pump forsupplying pressurized fluid to a plurality of devices. Because ofoperational and safety reasons, it becomes necessary for the controlsystem to incorporate some type of priority flow control to some of thedevices. For example, in a tractor which incorporates power steering andpower brakes, it is essential that at least one of these functions havepriority for any flow that is delivered from the pump so that theoperator is at all times in control of the vehicle.

In the past, the hydraulic control systems have normally been of eitherthe open-center system or the closed-center system. The open-centersystem incorporates a fixed displacement pump that is capable ofdelivering a maximum given flow required for the entire system and amain control valve associated therewith connects the pump directly tothe reservoir when the valve is in the neutral position. As expected ina system of this type, the pump is constantly delivering full flow atlow pressure when the control valve is in a neutral position whichresults in considerable energy loss.

Furthermore, in most hydraulic control systems of this type, the systemis designed to provide a maximum flow rate that is capable of operatingall of the fluid operated devices simultaneously. As can be appreciated,the maximum flow rate for the pump is seldom needed when operating avehicle of the type under consideration. Thus, in most instances, atleast a portion of the power is wasted since the system seldom demandsthe maximum flow of the pump and the excess flow is bypassed through apressure relief valve which means that further energy is lost.

The closed-center system incorporates a variable displacement, pressurecompensated pump capable of a given maximum flow rate and the system isalways operated at a predetermined maximum pressure. In this system,power is again wasted when less than the full pressure is required foroperating the devices to which pressurized fluid is being supplied. Inthis system, the pump is constantly operating at full pressure which mayresult in heating of the fluid when the system is in a neutral positionfor extended periods of time.

Quite recently a third type of system has been developed which may betermed as a pressure-flow compensating or "load-sensing" hydraulicsystem. This type of system is generally disclosed in U.S. Pat. No.3,486,334. The hydraulic circuit disclosed in this patent consists of avariable displacement pump, the output of which is controlled by apressure compensating member that is connected to the unit beingcontrolled so that the pump produces the required flow at the requiredpressure for delivery to the controlled unit. This type of hydrauliccircuit delivers low flow at low pressure for minimum pump wear, flowlosses and energy losses when the main control valves associatedtherewith are in a neutral position. The pressure-flow compensatedhydraulic system has the unique advantage of being capable ofmaintaining high pressures without delivering a large volume of fluidand also delivering large volumes of fluid at lower pressures as perapplication needs. In other words, this system combines the advantagesof the open-center system and the constant-pressure closed-center systemwhile avoiding the major disadvantages.

While the pressure-flow compensating hydraulic system has numerousadvantages over the other systems described above, to date such systemhas not found any widespread acceptance. At least one of the reasons forthe non-acceptance of such a system is the fact that in the past, moreexpensive control valves and directional valves were required in thehydraulic circuit utilizing the pressure-flow compensating pump.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a hydrauliccontrol system that includes a reservoir, a pump having pressurecompensating means associated therewith and first and second flowcontrol means controlling flow to first and second fluid operateddevices with a priority flow control valve in the system. The priorityflow control valve is designed to deliver all of the fluid from the pumpto the first fluid operated device whenever the pressure or the flowrequirements to that device are below a given level.

The priority flow control valve includes a housing having a valve borewith a valve spool slidable therein and cooperating therewith to definefirst and second chambers on the opposite ends of the spool. Thepriority flow control valve has an inlet port connected to the pump anda first outlet port connected to a first flow control means thatcontrols the flow to a first fluid operated device. A second outlet porton the priority control valve is connected to a second flow controlmeans which controls the flow of fluid to a second fluid operateddevice.

The valve spool of the priority flow control valve is normally biased toa first position by a spring in the first chamber and all of thepressurized fluid from the pump is delivered to the first outlet portwhen the valve spool is in the first position. The pressurized fluidreceived in the first outlet port is delivered to the second chamber onthe opposite end of the valve spool so that the spool is moved as afunction of the pressure in the first outlet port. A connection isprovided between the first fluid operated device and the pressurecompensating means so that the pump is operated as a function of thepressure and flow requirements demanded by the first fluid operateddevice. This connection is also in communication with the first chamberof the priority valve so that the pressure in the connection means orpilot conduit also controls the position of the valve spool in the valvebore.

In the specific embodiment illustrated, the first fluid operated deviceis a steering cylinder that forms part of the power steering system forthe vehicle while the second fluid operated device consists of theoperating means for the brakes that are normally associated with theleft and right-hand rear wheels of the vehicle so that flow priority isdeveloped to the steering system for the vehicle.

According to another aspect of the present invention, all of thenecessary check valves associated with the secondary or brake circuitportion of the hydraulic control system are housed in a single controlhousing that can readily be attached to the housing of a commerciallyavailable brake control valve, which greatly simplifies the constructionand assembly of the overall control system.

The hydraulic control system can also be designed to provide prioritycontrol flow to other fluid operated devices, as will be explained inmore detail later.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 schematically illustrates the hydraulic control system of thepresent invention with the priority flow control valve being shown incross-section;

FIG. 2 is a cross-sectional view of a portion of the hydraulic controlsystem that is associated with a commercially available brake valve;

FIG. 3 is a vertical sectional view as viewed along line 3--3 of FIG. 2.

DETAILED DESCRIPTION

While this invention is susceptible of embodiment in many differentforms, there is shown in the drawings and will herein be described indetail a preferred embodiment of the invention with the understandingthat the present disclosure is to be considered as an exemplification ofthe principles of the invention and is not intended to limit theinvention to the embodiment illustrated.

FIG. 1 of the drawings discloses a hydraulic control system, generallydesignated by the reference numeral 10, having the various features ofthe present invention incorporated therein. Hydraulic control system 10includes a reservoir 12 and a pump 14 having a pressure and flowcompensating means 15 associated therewith. Pump 14 is connected toreservoir 12 through a conduit 16 with the output of pump 14 deliveredthrough conduit 18 to an inlet port 20 of a priority flow control valve22. Priority flow control valve 22 includes a valve bore 24 extendingthrough housing 26 with opposite ends of the valve bore closed by plug28 and plate or plug means 30. A valve spool 32 is slidably supported invalve bore 24 and cooperates with housing 26 to define a first chamber34 at one end of spool 32 and a second chamber 36 at the opposite end ofspool 34. Biasing means or spring 40 is located in chamber 34 andnormally biases valve spool 32 to a first position, as will be describedin more detail later.

Valve housing 26 has a first outlet port 42 connected by conduit 44 toan inlet port 46 in a control valve means 48 that controls flow ofhydraulic fluid to and from a fluid operated device, such as a fluid ram50. Control valve means 48 is illustrated as being a steering valvewhich controls the flow of fluid to opposite ends of a steering cylinderand piston rod assembly that defines first fluid operated device 50.Steering valve 48 is connected by a return conduit 52 to reservoir 12and also has first and second outlet ports 54 and 56 respectivelyconnected by conduits 58 and 60 to opposite ends of the cylinder thatforms part of fluid ram 50. Thus, actuation of the control valve ineither direction from the neutral position will pressurize one of theconduits 58 or 60 while the second conduit will be connected toreservoir 12 through conduit 52.

Fluid ram 50 is also connected to a pressure compensating means 15associated with pump 14 through connection means which in theillustrated embodiment consists of a conduit 64 connected to a pilotport 66 in steering valve 48 with a unidirectional or ball check valve70 located in conduit 64 to prevent flow from pressure compensatingmeans 15 to fluid ram 50. Conduit 64 is also connected by branch conduit72 to the first chamber 34 having spring 40 therein while outlet port 42of priority flow control valve 22 is connected to second chamber 36through an opening 74 in valve housing 26.

Steering valve 48 may be a commercially available valve sold by RossEngineering under part No. HGA12006 with minor modifications of suchvalve to provide a pilot port which is connected to either end of thecylinder of the fluid ram 50 when the valve is actuated in eitherdirection from the neutral position. Alternatively, steering valve 48may be the above commercial valve without modifications in which caseconduits 58 and 60 would be connected directly to conduit 64 withsuitable check valve means associated therewith such as check valves 94,96 to be described later to prevent flow between the two conduits 58 and60 with a connection to check valve 70 between the two valves to preventflow from conduit 64 to either conduit 58 or 60.

Control valve housing 26 has a second outlet port 76 connected byconduit 78 to an inlet of second flow control means or brake valve 80.Brake valve 80 is a commercially available valve sold as part No.39300-AD by Cessna Corporation. Brake valve 80 has first and secondmanually actuated valve spools 82 and 84 which, when actuated,respectively supply pressurized fluid through conduits 86 and 88 to theleft and right-hand brakes (not shown). In the neutral position for therespective valve spools 82 and 84, conduits 86 and 88 are connected toreservoir 12 through a return conduit 90.

The brake circuit of the hydraulic control system also includes ahydraulically actuated differential lock of the commercially known typewhich is capable of interlocking the two wheels. This differential lockis hydraulically actuated by supplying pressurized fluid from a separatesource to a control valve (not shown). The differential lock is alsointerconnected with the hydraulic circuit for the left and right-handbrakes so that the differential lock is automatically released wheneither of the brakes is actuated and both brake conduits are connectedto pressure compensating means 15 so that pump 14 may be operated as afunction of the pressure and flow requirements in either the left orright-hand brake.

According to one aspect of the present invention, the interconnectionmeans between the brakes and the differential lock as well as thepressure compensating means 15 are interrelated to substantiallysimplify the number of parts and construction thereof required forproviding such interconnection.

As illustrated in FIG. 1, conduits 86 and 88 are interconnected by aconduit 92 that has two oppositely directed unidirectional valves 94 and96 located therein. These valves prevent flow between left andright-hand conduits 86 and 88 but will accommodate flow from eitherconduit to a differential lock conduit 98 connected to conduit 92between the two unidirectional check valves 94 and 96. A further branchconduit 100 leads from between the two check valves 94 and 96 topressure compensating means 15. Conduit 100 has a unidirectional valve102 therein which blocks flow from compensating means 15 when the brakeor secondary circuit is in a neutral condition. Also, conduit 100 has apressure responsive check valve 104 located therein and this pressureresponsive check valve is opened in response to pressurization of thefluid is conduit 78 leading from priority valve 22 to brake valve 80.Whenever conduit 78 is pressurized, check valve 104 is held in an openposition to provide a direct connection from conduit 92 to pressurecompensating means 15 through unidirectional valve 102. However, if thefluid is conduit 78 is not pressurized, check valve 104 will be closedand will prevent flow through conduit 100. This allows the operator toprovide manual braking power by manual pressurization of the fluid ineither conduits 86 or 88 to the respective brakes should hydraulic powerbe lost.

According to one aspect of the invention, check valves 79, 94, 96, 102and 104 are all incorporated into a single housing that can readily beattached to the commercially available brake valve 80 to convert thebrake valve to a control valve means that can be utilized with apressure compensating pump. This arrangement simplifies the constructionof the system and reduces the overall cost of the system.

The details of the valve attachment are illustrated in FIGS. 2 and 3.The valve attachment consists of a valve housing 120 that may beconnected directly to the housing of brake valve 80 by bolts 121 andhousing 120 has an inlet bore 122 extending therethrough with anenlarged portion 124 at one end thereof. The enlarged portion 124supports a ball check valve 126 that is biased to a closed position by aspring 128 and defines unidirectional valve means 79, illustrated inFIG. 1. A suitable fitting 129 is threaded into the opposite end of bore122 for providing a connection to conduit 78 leading to housing 120.Inlet bore or port 122 has a transverse passage 130 extending therefromwith a rod 132 slidably supported in passage 130. The opposite end ofpassage 130 has an enlarged portion 130a which receives a threadedfitting 134 therein that is capable of having conduit 98 connectedthereto for connection to the circuit for the differential lock (notshown). A spring biased spool 136 is biased by a spring 138 intoengagement with a value seat defined at the end of an enlarged portion130a of passage 130. Rod 132 and valve element 136 define pressureresponsive valve means 104 illustrated in FIG. 1.

Valve passage 130 has a further intermediate sized portion 142 that isin communication with a further bore 144 having a threaded fitting 146at the outer end thereof for connection of conduit 100 thereto. Afurther spring biased valve element 148 is biased into engagement with avalve seat by a spring 149. Vavle element 148 and spring 149 definepilot check valve 102 illustrated in FIG. 1.

The enlarged portion 130a of passage 130 is also connected by passagemeans 150 to a further passage 152 that extends from one end of housing120 to the opposite end. Passage 152 is closed at both ends byrespective plug members 154 and 156 and has a freely movable ball 158located between the inner ends of the respective plugs. Each plug has anaxial opening 160 and a transverse opening 162 with the transverseopenings being in communication with perpendicular bores 164 and 166.Bores 164 and 166 extend to the surface of valve housing 120 and havefittings 168 threadedly received therein (only one being shown in FIG.3). These fittings 168 respectively have conduits 86 and 88 connectedthereto. Ball 158 and associated passages 150, 152 define bothunidirectional valve means 94 and 96 and conduit 92, illustrated in FIG.1.

The operation of the circuit so far described will now be summarized.Assuming that steering valve 48 and valve spools 82 and 84 of brakevalve 80 are all in the neutral position, there will be no demand forpressurized fluid from pump 14. The pump 14 will then have a minimumflow output at a very low pressure, such as 200 p.s.i. to replenish anyfluid leakage within the control system. In this condition, spring 40will bias valve spool 32 to the first position wherein the flow of fluidfrom inlet port 20 to second outlet port 76 is blocked. It should benoted that the spring force for spring 40 is such that it isapproximately equal to the fluid force on valve spool 32 at the minimumlevel of 200 p.s.i. Also any flow from first outlet port 42 is blockedby control valve 48.

If a steering function is to be performed, steering valve 48 will beactuated so that pressurized fluid is being demanded either in conduit58 or conduit 60 while the other conduit will be connected to reservoir12 through conduit 52. When this condition occurs, the conduit to bepressurized will be connected internally of valve 48 to pilot port 66and the pressure of the fluid in such conduit will be sensed by pressurecompensating means 15. The pressure compensating means 15 will actuatepump 14 to provide the necessary flow of fluid at the required pressurethrough inlet port 20, valve bore 24, outlet port 42 through valve 48 tothe appropriate end of the cylinder that forms part of fluid ram 50. Atthe same time, the pressure of the fluid in port 42 is delivered throughopening 74 to the second chamber 36 to oppose spring force 40 while thepressure of the fluid in conduit 64 is delivered to first chamber 34 toassist spring 40 in maintaining the valve spool in the first position.Thus, the pressure of the fluid in outlet port 42 must reach a certainlevel before the valve spool can be moved from the first position, tothe right as viewed in FIG. 1, to allow fluid under pressure to bedelivered to the secondary port 76. Stated another way, valve spool 32is normally held in a first position, when hydraulic fluid is demandedby steering valve 48 to block flow to the secondary circuit consistingof brake valve 80 and the movement of the valve spool from the firstposition is controlled by the pressure of fluid in outlet port 42, thepressure of fluid in the pilot conduit 64 as well as the spring force40.

Assuming now that all of the pressure and flow requirements forproducing the desired steering functions are met, the pressure of thefluid in outlet port 42 and opening 74 as well as chamber 36 willincrease sufficiently to move the valve spool 32 to the right as viewedin FIG. 1 and, therefore, connect inlet port 20 to second outlet port 76so that pressurized fluid from pump 14 is available for actuating eitherthe left or right-hand brake. Assuming now that one of the brake spools82 or 84, such as brake spool 82, is actuated, such actuation willproduce a demand for pressurized fluid in conduit 86. This demand forpressurized fluid will be sensed by pressure compensating means 15through check valve 94, pressure responsive valve 104 (which is now heldopen by the pressure of fluid in conduit 78), check valve 102 andconduit 100. The drop in pressure within conduit 86 will produce ademand for increased pump output from pump 14 so that pump 14 willproduce the desired amount of fluid at the required pressure foractuating the brake.

If there is no demand for pressurized fluid to steering valve 48, anypressure demands for fluid to conduit 78 will also be delivered throughopening 74 to chamber 36 to cause valve spool 32 to move to the right,as viewed in FIG. 1, to a position where ultimately first outlet port 42will be blocked by valve spool 32 and all of the fluid from pump 14 willbe delivered to second outlet port 76 and ultimately to brake valve 80.

Thus, it will be appreciated from the above description that the presenthydraulic control system provides a very simple arrangement forproviding priority flow to one of two fluid operated components. Ifdesired, this same system can also be utilized to provide pressurizedfluid for various other functions. For example, as illustrated in FIG.1, conduit 78 could be connected to a further auxiliary control valve200 which could be connected to reservoir 12 by a conduit 202 and topressure compensating means 15 through a conduit 204 having aunidirectional valve 206 therein. The auxiliary valve could be part ofthe circuit that provides pressurized fluid to hydraulic components,such as the draft control system for the vehicle or pressurized fluid totrailing implements, such as tilling devices which are hydraulicallyactuated. If desired, a pressure regulating valve could be locatedbetween conduit 78 and control valve 200 to prevent flow to controlvalve 200 when the pressure of the fluid in conduit 78 is below acertain level required for actuation of the brakes. This would insurepriority of flow to the secondary circuit consisting of brake valve 80before fluid is available for operating the auxiliary circuit.

It is also contemplated within the spirit of this invention that afurther control valve 200 could be replaced with a further prioritycontrol valve 22 and, therefore, two additional hydraulic circuits couldbe controlled with a single pump. Again, priority of flow could beestablished for one of the two circuits as explained with respect to thebasic hydraulic control circuit disclosed herein.

The same hydraulic control system can also be utilized for supplyingpressurized fluid to other fluid operated circuits which require verylow pressure. For example, hydraulically actuated transmissions of thetype incorporated into many agricultural tractors of the type underconsideration can be actuated by fluid pressure that is pressurized toless than 200 p.s.i. Thus, in the control system illustrated anddescribed above, the conduit 18 could be connected directly to atransmission circuit 210 and provide the necessary pressurized fluid foractuating the various hydraulic components in the transmission. Thisarrangement would not in any way deter from the priority of flow to thesteering valve 48 since any demand for fluid to the steering valve wouldautomatically increase the pressure in conduit 18.

It should also be realized that while the two fluid operated deviceshave respectively been illustrated as a steering ram 50 and left andright-hand brakes, the system could readily be designed for providingpriority flow to any particular function, such as a brake circuit withother functions, or a steering system with other functions such as aremote hydraulic system associated with an implement and a draft controlcircuit that controls the movement of a hitch on the tractor.

What is claimed is:
 1. A hydraulic control system including a reservoir,a pump having pressure compensating means associated therewith, firstand second flow control means controlling flow to first and second fluidoperated devices and a priority flow control valve between said flowcontrol means and said pump, said priority flow control valve includinga housing having a valve bore therein with a valve spool slidable insaid valve bore and cooperating therewith to define first and secondchambers on opposite ends of said spool, biasing means in said firstchamber biasing said spool to a first position, said housing having aninlet port connected to said pump and communicating with said bore andfirst and second outlet ports respectively connected to said first andsecond flow control means, said first outlet port being in communicationwith said inlet port when said valve spool is in said first positionwhile said valve spool blocks flow to said second outlet port, meansconnecting said first outlet port to said second chamber, connectionmeans between said first fluid operated device and said pressurecompensating means providing pilot pressure flow from said first fluidoperated device to said pressure compensating means when said firstcontrol means is in either actuated position to operate said pump as afunction of the pressure of fluid in said first fluid operated device,said connection means being connected to said first chamber so that saidvalve spool is moved as a function of the pump pressure, the pilotpressure and the biasing means, unidirectional valve means in saidconnection means preventing flow from said pressure compensating meansto said first chamber and said first fluid operated device, said secondfluid operated device including first and second fluid operated brakeactuating means with first and second conduits connecting said first andsecond fluid operated brake actuating means to said second flow controlmeans, a branch conduit between said first and second conduits having apair of opposed unidirectional valves therein preventing flow betweensaid first and second conduits and unidirectional valve means connectedto said branch conduit between said opposed unidirectional valves and tosaid pressure compensating means for preventing flow from said pressurecompensating means to said first and second fluid operated brakeactuating means.
 2. A hydraulic control system as defined in claim 1, inwhich said second flow control means includes a brake valve having ahousing with a brake inlet, further including a housing attachmentconnected to said brake valve and having a bore extending therethroughand connected at one end with said second outlet port and at theopposite end connected to said brake inlet, said bore having a springbiased ball check valve therein preventing flow from said brakeactuating means to said pressure compensating means, said housingattachment having passage means extending from said bore and connectedto said first and second fluid operated brake actuating means and a ballfreely movable in said passage means and having first and secondpositions, said ball defining said opposed unidirectional valves andsaid passage means defining said branch conduit with said unidirectionalvalve means connected to said passage means between said bore and saidball.
 3. A hydraulic control system as defined in claim 2, furtherincluding pressure responsive valve means in said passage means betweensaid unidirectional valve means and said ball, said pressure responsivevalve means being opened in response to pressurized fluid in said bore,an additional conduit leading from said passage means between said balland said pressure responsive means.
 4. A hydraulic control system asdefined in claim 1, in which said second outlet port of said priorityflow control valve is also connected to an auxiliary valve forcontrolling flow to auxiliary equipment.
 5. A hydraulic control systemas defined in claim 4, in which said pump is connected to ahydraulically actuated transmission control system for supplying fluidthereto.